The present invention relates generally to the field of medical devices and specifically to medical devices which are designed to monitor the respiratory characteristics of patient breathing, especially those patients attached to mechanical ventilation systems. Persons who have suffered traumatic injury or some debilitating disease may have to be placed on mechanical ventilation systems. When a patient is on such a system it is for a doctor or other medical professional to gather data to assess whether a patent is able to breathe without the aid of the mechanical ventilation device. Monitoring of a patient's respiratory characteristics while the patient is connected to a mechanical ventilation system can be accomplished through the use of pulmonary mechanics techniques.
The term pulmonary mechanics refers to the graphical monitoring of a patient's lung and breathing performance. Pressure values corresponding to the ambient pressure in a patient's airways and to the volumetric flow of respiratory gases between the patient and a mechanical ventilator are measure and recorded. These values are then used to calculate and graphically display pulmonary mechanics parameters that assist the doctor or respiratory therapist in assessing, for example, a patient's pulmonary compliance, tidal volume, and work of breathing. This type of monitoring helps a doctor or respiratory therapist assess the patient's respiratory condition so that the ability of a doctor or other medical professional to intervene prior to the onset of respiratory fatigue or failure is improved.
In addition, this type of monitoring is very useful in making fine adjustments to a mechanical ventilating system in order to optimize a patients ventilation. Pulmonary mechanics monitoring is also very useful in assessing whether and how a patient might be weaned from a mechanical ventilation system.
Prior to this invention there have been many different types of technologies utilized to accomplish respiratory monitoring. The most prevalent technology is use of a differential pressure flow meter. Differential pressure flow meters function by measuring a pressure drop across a restriction placed in an airway. This pressure drop is related to the flow rate of respiratory gases flowing through the airway in which the obstruction is placed. Using empirical methods a relationship can easily be established between the drop in pressure across a restriction in an airway and the amount of flow through that airway. Combining the flow rate through an airway with the ambient pressure within that airway allows a doctor or respiratory therapist to quickly assess a patient's ability to breathe on their own or to assess the performance of a ventilation system.
While respiratory monitoring systems in use in hospitals today are generally able to perform adequately, they are relatively large, complex, expensive to manufacture, sensitive to temperature fluctuations, and prone to quantization errors. Because the primary function of any mechanical ventilation system is to provide respiratory gases to a patient, all restrictions to flow of respiratory gases in the ventilation system must be minimized. This results in the need to use differential pressure type flow sensors that also minimize any restrictions to flow. But because the magnitude of a differential pressure signal is directly proportional to the magnitude of the flow restriction used in the differential pressure flow sensor, the situation arises where a relatively large range of flow rates within the ventilation system will produce a correspondingly small range of differential pressure signals. Where the analog-to-digital converter (A/D) used to convert analog signals from a differential pressure transducer to complimentary digital values has a limited dynamic range, quantization error will be introduced into the data stream of the differential pressure flow meter. Quantization error is defined in this context as an error relating to the assignment, by an A/D, of one digital voltage value to two distinct analog voltage signals representing different flow rates within the ventilation system's airways.
In attempting to remedy this quantization error some prior art devices created complicated variable gain systems to variably amplify voltage signals representative of flow rates within a ventilation system in such a way as to overcome the limited dynamic range of the A/ID converts available. However, these variable gain systems require numerous signal amplifiers making them harder to manufacture and ultimately increasing the end cost of the devices. Using a greater number of amplifiers also increases the possible amount of error in the device's output due to amplifier offset and gain differences.
Another problem associated with differential pressure type flow sensors is that of error introduced into the system due to temperature changes. Because pressure sensing devices such as a differential pressure transducer are sensitive to temperature in addition to pressure, any change in the temperature of the pressure transducer itself can cause critical variances in the resulting data. Even in temperature compensated transducer models, residual variations can cause unacceptable variances in the data.
Therefore there is a need for a respiratory function monitoring device that reduces or eliminates quantitisation errors over its useful range. A further need is recognized for a respiration monitoring device capable of calculating pulmonary mechanics parameters in a manner that is independent of any variations in temperature. Yet another need recognized is for an electronics package capable of performing pulmonary mechanics respiratory monitoring that is small, inexpensive and useable in a wide range of existing monitoring platforms including hand-held monitors and bedside cart-mounted monitors.
Therefore, it is an object of this invention to provide a flow sensor that has a low resistance to air flow therethrough and which also provides a more linear differential pressure output in response to low air flow rates.
It is another object of this invention to provide a wave form analyzing device capable of transducing the pressure signals derived from a flow sensor that has a high resolution output having little or no quantization error.
It is yet another object of this invention to provide a wave form analyzing device capable of transducing the pressure signals derived from a flow sensor that is immune from temperature induced drift in the output of the pressure transducers used to transduce the pressure signals from the flow sensor.
It is yet another object of this invention to provide a structure for purging the fluidic connections between the flow sensor and the wave form analyzer of blockages and foreign materials.
A final object of this invention is to provide methods for implementing an auto zero function and a purging function as well as for calibrating the wave form analyzing device and the flow sensor.
The present invention is an improvement on the above noted technology. The inventors know of no prior art that teaches or discloses the subject matter of the invention as claimed and described herein.